Thiolated hyper-crosslinked tetraphenylborate with tunable selectivity for sequential recovery of Pd and Pt from autocatalyst leachate

Abstract

Palladium (Pd) and platinum (Pt) recovery is crucial due to their extensive applications, economic value, and limited availability. This study developed a thiolated hyper-crosslinked adsorbent (SH-x-TPB) using a tetraphenylborate-based matrix grafted with sulfonic acids, which were subsequently reduced to thiols (−SH). The SH-x-TPB is highly stable in acidic environments, making it suitable for harsh feed conditions. It is also highly porous (63 %), with a hierarchical structure (91 % mesopores) and a high BET surface area (>400 m2 g−1). SH-x-TPB exhibits exceptionally rapid sorption rates for Pd2+ (983 g mmol−1 hr−1) and Pt4+ (31 g mmol−1 hr−1), with high capacities of 386.96 mg Pd2+ g−1 and 72.59 mg Pt4+ g−1. Characterization indicates that Pd2+ adsorption, typically as square planar PdCl42-, is non-ideal and heterogeneous, involving ligand substitution, stable π-complexation, and reduction to Pd⁰. In contrast, Pt4+ adsorption as octahedral PtCl62- is strictly Langmuir-type and homogeneous, primarily involving Pt4+ reduction to Pt2+ by −SH groups. Sorption is pH- and time-dependent, favoring Pd2+ at pH 1–2 within 10 min and Pt4+ at pH 3–4 within 60 min. Under these conditions, a two-stage sequential recovery process successfully collected Pd2+ first, followed by Pt4+, achieving overall recoveries of 99.37 % and 99.67 %, respectively, while leaving unwanted ions in the leachate. The captured metals can be desorbed with 1 M thiourea/2 M HCl solution, rendering the reactivated SH-x-TPB reusable with consistent performance. The competitive adsorption capacities, selective recovery capabilities, recyclability, and simple fabrication methods of SH-x-TPB make it a promising material for sequential metal recovery applications.